1. Field of the Invention
This invention relates to optical polarizing devices generally, and more specifically to rugate polarizing beamsplitters for narrow wavelength bands.
2. Description of the Related Art
A common optical element known as a xe2x80x9cMacNeille polarizerxe2x80x9d includes two right angle prisms, one of which has a multilayer dielectric coating on its hypotenuse face. The two prisms are bonded together with an optical quality cement to form a cube, with a planar refractive transition slicing diagonally across the cube where the two prisms are joined on a face. The transition partially reflects and partially transmits incident light, thereby splitting a beam. At the transition a plane of incidence is defined by two vectors, one in the direction of light propagation, and the other normal to the surface. The component of the light""s e-field in the plane of incidence is referred to as the xe2x80x9cP-polarizationxe2x80x9d component; the component in a plane perpendicular to the plane of incidence is referred to as the xe2x80x9cS-polarizationxe2x80x9d. By choosing the proper angle for given prism materials with different refractive properties, it is possible to satisfy well known xe2x80x9cBrewster conditionxe2x80x9d, which causes the P-polarized components to be transmitted while the S-polarization is reflected. Thus, a beam can be split into S and P polarized beams.
A more recent refinement of the polarizing beamsplitter is described in U.S. Pat. No. 5,828,489 to Johnson et al. (1998). This patented beamsplitter uses a gradient index film, specifically a xe2x80x9crugatexe2x80x9d filter, having an index of refraction which oscillates with depth into the film. The rugate filter is embedded in an optical medium at an angle with respect to an incident beam. This patented beamsplitter arrangement efficiently reflects S-polarized light at specific narrow wavelengths, while transmitting S-polarized light at other wavelengths (and P-polarization at all wavelengths). This property is often useful when working with narrow band optical sources (such as lasers).
The rugate polarizing beamsplitter of the Johnson patent can only operate within certain constraints, however. For best polarization selection and narrow pass-band, the device requires that the incident radiation be very nearly parallel, plane waves propagating in a single direction, without divergence of convergence. This requirement results from a limitation: First, for polarization separation, the angle of incidence must match the Brewster angle (given the high and low index of refraction of the rugate filter coating or the high and low index of refraction of the layer stack in the MacNeille polarizer). As a result of this constraint, the index of the embedding medium limits the choice of the beamsplitter""s usable geometry. Second, the chromatic pass-band of the rugate filter is sensitive to the angle of incidence. Therefore, ideal pass and reflection characteristics are only approachable at a unique angle of incidence (for a given filter).
FIG. 1 shows a prior art narrow wavelength polarizing beamsplitter (adapted from the Johnson patent) in the path of a divergent light beam. The view is simplified to show only one dimension of divergence, but the principle is representative of the situation with any divergent beam. Assume that the chief ray 10 enters the cube 12 as shown, and is incident on the rugate film 14 at angle xcfx86 (to the normal 16). For best operation, the period of rugate film and the angle xcex8 of the rugate can be chosen appropriately, according to the teachings of the Johnson patent and known optical principles. However, it should be apparent that due to divergence of the beam, other rays such as to Top ray 18 and Lower ray 20 which strike the rugate at some distance from the optical center (intersection of the chief ray with the rugate) will have angles of incidence which differ from xcex8. Thus, in the prior art device the rugate period and the angle of incidence will not be optimized for all rays of a divergent beam.
A previously described rugate polarizing beamsplitter is thus not able to satisfy the requirements of wide angle operation, while maintaining narrow band selection and polarization selectivity.
In view of the above problems, the present invention is a polarizing beamsplitter which includes an optical medium and a rugate filter embedded therein. The beamsplitter efficiently reflects S-polarized light at specific narrow wavelengths, while transmitting S-polarized light at all other wavelengths (and P-polarization at all wavelengths).
In a first embodiment, the invention includes a rugate filter arranged in the path of a beam in such a way that the angle of beam propagation within the rugate filter is substantially equal to 45 degrees, notwithstanding any variations in incident angle across the rugate. For the special case of a collimated beam, this requirement can be satisfied by orienting the rugate filter at an angle to the beam, wherein said angle satisfies Snell""s law given the average index of the rugate, the index of the embedding optical medium, and the requirement that the angle of the beam within the rugate is 45 degrees. The index of the optical medium need not match the average index of the rugate. For the more general case of a divergent (or convergent) beam, the first embodiment includes a rugate filter with an average refractive index which varies with position on the rugate plane in the non-thickness direction. The variation in the rugate""s average index compensates for variations in incident angle across the light beam (for example, across a divergent cone).
In the second embodiment, the period of the rugate varies across the surface of the rugate, while the average index of the rugate remains substantially constant across the surface (does not vary with position in the non-thickness directions). A predetermined variation in the period is imposed to compensate for variations in incident angle across the light beam (for example, across a conically divergent beam).
More generally, an unlimited number of variations of the invention are possible which vary both the average rugate index and the rugate period as functions of position on the rugate surface, according to a predetermined relation chosen to compensate for variations in incident angle across light beam.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: